The road to gigantism: understanding how the ancestors of giant sauropod dinosaurs grew

The gigantic sizes that long-necked quadrupedal herbivorous dinosaurs (derived sauropodomorphs or sauropods) grew to, have captivated the imagination for decades. Analyses of the microstructure of fossil bones of these creatures, have shown that they attained these massive sizes through largely uninterrupted rapid deposition of bone, with growth marks (think of rings in a tree) restricted to the outer regions of the bone, i.e. constant sustained growth until the attainment of skeletal maturity.

In contrast, the ancestors of these dinosaurs (basal sauropodomorphs), were smaller and more bipedal animals (inclined to walk on two legs). These animals exhibited slower cyclical periods of growth, followed by regular (potentially seasonal) pauses in growth indicated by growth marks – similar to the way a layer cake would be interrupted by layers of icing. Up until recently, quadrupedal sauropods and bipedal basal sauropodomorphs growth dynamics were thought of as existing in a dichotomy between slower cyclical growth and rapid sustained growth strategies.

Recently published work in the journal The Anatomical Record,on the bone microstructure of the 235 million year old basal sauropod dinosaur Antetonitrus ingenipes from South Africa by PhD candidate Emil Krupandan from the Biological Sciences Department at the University of Cape Town in conjunction with his supervisors Professor Anusuya Chinsamy-Turan and Dr. Diego Pol (from the Museo Paleontológico Egidio Feruglio in Trelew, Argentina) shows that en route to the attainment of gigantism species that lie between smaller basal sauropodomorph dinosaurs and larger quadrupedal sauropods ,such as Antetonitrus, exhibited a unique pattern of growth, intermediate between basal and derived sauropodomorphs.

“What we’re seeing in Antetonitrus, is that these dinosaurs show a mosaic of rapid bone growth, with erratic placement of growth marks and textural shifts in the organisation of fibro-lamellar bone that are not quite as regular as their ancestors - essentially were seeing an intermediate growth strategy with elements of both basal and derived sauropodomorph growth strategies present in these early sauropods.”, says lead author Emil Krupandan.

Krupandan says that, “Recent, new work by other authors, on related members of this group of derived sauropod ancestors, called Lessemsauridae, also finds similar growth patterns in these close relatives. We’re seeing that as early as the Late Triassic the steps towards gigantism were emerging, as evidenced by increases in bone growth rates of these creatures. It’s also important to note that this group shows a unique growth strategy. In addition, based on the gross anatomy of these related sister dinosaurs (Ingentia prima from Argentina and Ledumahadi mafube also from South Africa) they also exhibited different anatomical adaptations towards large size and quadrupedalism compared to the columnar stance of their latter relatives like Diplodocus or Argentinosaurus.”.

Advances in our understanding of bone growth and anatomy in this group of dinosaurs shows us that the transition from small bipedal basal sauropodomorphs to massive quadrupedal sauropods was anything but clear-cut, with many experiments in growth and morphology along the way.